This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. A kind of high efficacy lamp is the arc discharge metal halide lamp that is being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light-transmissive arc discharge chamber sealed about an enclosed pair of spaced apart electrodes and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation. Their superior performance with respect to other kinds of high pressure arc discharge lamps in measures such as luminous efficiency, color rendering and color stability is responsible for their increasing use.
The better performance of these lamps is due to the higher operating temperatures possible for the ceramic arc discharge tubes ceramic material than can be achieved with lamps using quartz material arc tubes, as well as the more precise dimensional control that is possible with ceramic tubes formed with sintered powders previously compacted in molds providing for preformed openings for electrodes to be inserted than for quartz tubes formed from an oxide that is heated to have a viscosity allowing it to be pressed against the electrodes provided therewith. The seal obtained between a polycrystalline alumina (PCA) ceramic tube and the two spaced apart access electrodes each extending from the enclosed space in the tube interior formed by its bounding walls to the tube exterior is critical to the successful operation over substantial periods of time for this lamp in view of the extreme conditions occurring in this interior space during lamp operation.
High pressure sodium lamps utilize niobium as the electrode material for the discharge chamber access electrodes extending between the chamber interior and the region outside the chamber since its thermal coefficient of expansion (TCE) is well matched to that of polycrystalline alumina. Such electrodes are joined to the polycrystalline alumina by a ceramic sealing frit formed of mixed metal oxides having a thermal expansion coefficient similar to both that of polycrystalline alumina and niobium. This sealing frit is also resistant to sodium based corrosion at the high temperatures encountered in the discharge chamber during lamp operation.
However, this arrangement is not suitable for metal halide lamps having ceramic arc discharge chambers since the salts of the halides therein are corrosive to both niobium and the sealing frit used, this being so even with such discharge chambers being operated at the lower cold spot temperatures usual for metal halide lamps because of the greater chemical activity of halides. Consequently, a variety of alternative arrangements have been tried as possible bases for overcoming the sealing problem involving access electrodes in ceramic arc discharge tubes used in metal halide lamps.
Refractory metals, such as molybdenum, tungsten, platinum, rhodium, rhenium, etc., are resistant to halide corrosion during lamp operation and may be used as materials for access electrodes. They, however, typically have lower corresponding thermal coefficients of expansion than that of polycrystalline alumina as shown in the Table below. As a result of thermal cycling during each lamp operation and over the operating life of the lamp, such large differences between the thermal coefficients of expansion of the access electrodes and the ceramic material in the arc discharge tube body leads to separations between the metallic access electrodes and the ceramic arc discharge tube bodies in which they positioned. These separations can cause seal fracture leaks of the vapors in the arc discharge tube enclosed space, and even fractures of the tube itself near these electrodes thereby leading to loss of arc discharge tube hermeticity.
TABLEThermal Coefficients of Expansion of Commonly Usedor Possibly Used Metal Halide Lamp MaterialsApproximate Thermal Coefficients ofMaterialsExpansion Values (μm/m/K)Alumina8.0Aluminum nitride5.4Niobium8.0Molybdenum6.0Tungsten5.2
In general, sealing methods for sealing access electrodes in the arc discharge tube body can be divided into four categories—use of a sealing frit, sintering the tube body about the electrode, use of graded thermal expansion coefficient seals that substantially match the thermal expansion coefficient of the electrode on one side thereof and that of the body on the other side, and use of altogether new arc tube materials. Some of the methods within these categories overlap in practice (for example, the use of graded plug material to effect a seal by sintering).
A typical ceramic arc discharge tube, 20, in present use for a ceramic metal halide lamp formed about an enclosed, or contained, region as a preformed shell structure is shown in FIG. 1, this enclosed region containing various ionizable materials, including metal halides and mercury which emit light during lamp operation and a starting gas such as argon or xenon. In this structure for tube 20, a pair of polycrystalline alumina, relatively small inner and outer diameter truncated cylindrical shell portions, or capillary tubes, 21a and 21b, are each concentrically joined to a corresponding one of a pair of polycrystalline alumina end closing disks, 22a and 22b, about a centered hole therethrough so that an open passageway extends through each capillary tube and through the hole in the disk to which it is joined. These end closing disks are each joined to a corresponding end of a polycrystalline alumina tube, 25, formed as a relatively large diameter truncated cylindrical shell, to be about the enclosed region to provide the primary arc discharge chamber. These various portions of arc discharge tube 20 are formed by compacting alumina powder into the desired shape followed by sintering the resulting compact to thereby provide the preformed portion, and the various preformed portions are joined together by sintering to result in a preformed single body of the desired dimensions.
Thus, there results two pathways from regions outside arc discharge tube 20 into the primary chamber region enclosed within ceramic arc discharge tube 20, each along a corresponding one the passageways having a selected diameter and extending through the preformed capillary tubes and end closing disks. The passageways thus formed are each to accommodate a corresponding access electrode arrangement. This configuration results in lower temperatures in the sealing regions in the capillary tubes during lamp operation since the ends of the electrode arrangements extend through the capillary, or electrode tubes, into the enclosed chamber a significant distance thereby spacing them, and the discharge arc established between them, further from the seal regions in the electrode tubes at the ends of discharge tube 20.
The electrode arrangement in each of these passageways is provided in three parts including in the left electrode arrangement a small diameter outer part niobium rod, 26a, surrounded by a ceramic sealing frit, 27a, in electrode tube 21a except where joined to the middle part molybdenum or cermet rod, 29a, by a butt weld, this niobium rod extending from that electrode tube to the outside of arc discharge tube 20. In the right electrode arrangement, there is included a small diameter outer part niobium rod, 26b, surrounded by a ceramic sealing frit, 27b, in electrode tube 21b except where joined to the middle part molybdenum or cermet rod, 29b, by a butt weld, the niobium rod similarly extending from that electrode tube to the outside of the arc discharge tube 20. At the other end of the left electrode arrangement, a small diameter inner part tungsten rod, 31a, is positioned adjacent one end of rod 29a and extends from electrode tube 21a into the enclosed region of arc discharge tube 20. An electrode coil, 32a, is mounted on the end of rod 31a in the enclosed region of arc discharge tube 20. Similarly, at the other end of the right electrode arrangement, a small diameter inner part tungsten rod, 31b, is positioned adjacent one end of rod 29b and extends from electrode tube 21b into the enclosed region of arc discharge tube 20. An electrode coil, 32b, is mounted on the end of rod 31a in the enclosed region of arc discharge tube 20.
Since tungsten rods 31a and 31b, with electrode coils 32a and 32b mounted thereon, respectively, must be positioned in the corresponding one of electrode tubes 21a and 21b, and extend into the enclosed region in arc discharge tube 20, after the fabrication of arc discharge tube 20 has been completed, the diameter of the passageways extending through the preformed electrode tubes and end closing disks must have inner diameters exceeding the outer diameters of the corresponding one of electrode coils 32a and 32b. As a result, there are substantial annular spaces between the outer surfaces of tungsten rods 31a and 31b and the inner surfaces of electrode tubes 21a and 21b which are taken up in part by the provision of molybdenum coils, 34a and 34b, around and against corresponding portions of tungsten rods 31a and 31b, and which also extend to be around and connected to corresponding portions of rods 29a and 29b, to complete the interconnections thereof and reduce the condensation of the metal halide salts in these regions. These interconnections could also be provided by butt welds. Thus, a right electrode arrangement, 35a, and a left electrode arrangement, 35b, result.
Electrode arrangements 35a and 35b have “compromise” properties components in the seal regions, these being outer part niobium rods 26a and 26b which provide very good thermal expansion matching to the polycrystalline alumina but which are also subject to chemical attack during operation by the metal halides within arc discharge tube 20. The exposure length of each of these outer parts within arc discharge tube 20 must be limited thus requiring the presence of the bridging middle part of the electrode arrangement, usually a molybdenum or cermet rod, between it and the tungsten electrode. Care is also taken to ensure that the melted sealing frits flow completely around and beyond the niobium rods thereby forming a protective surface over the niobium against the chemical reactions due to the halides. The frit flow length inside the capillary tube needs to be controlled very precisely. If the frit length is short, the niobium rod is exposed to chemical attack by the halides. If this length is excessive, the large thermal mismatch between the frit and the solid middle part molybdenum, tungsten or cermet rod beyond the niobium rod leads to cracks in the sealing frit or polycrystalline alumina in that location. These electrode arrangements with a complex construction requiring butt welds or crimpings therealong, also demand strict monitoring of the sealing process as indicated above. If the niobium could have some other material substituted therefor at the seal location, the electrode fabrication and the subsequent sealing process used therewith can be simplified and made more resistant to halide based chemical corrosion during operation as well.
Ceramic sealing frits 27a and 27b of mixed metal oxides are more halide resistant than the ones used in high pressure sodium lamps in effecting the seals between the polycrystalline alumina of the corresponding electrode tube and the corresponding niobium rod. However, while resistant, this sealing frit is not impervious to chemical attacks. Thus, elimination of niobium at the seal location would make possible a minimum and non-critical exposure length for the sealing frit within the electrode tubes
In these circumstances, of course, other ceramic arc discharge tube constructions for ceramic metal halide lamps that make use of different sealing methods have been used. These include methods such as direct sintering of polycrystalline alumina to the electrode arrangement, the use of cermets and graded thermal coefficient of expansion seals, or even the use of new arc tube materials that enable straight sealing of the tube body to a single material electrode such as molybdenum or tungsten. There have been occasional introductions of lamps that used a cermet to replace niobium. But these alternative methods have not yet been able to demonstrate an overall advantage with respect to improved lamp performance, lower cost, or compatibility with existing lamp factory processes.
In a further alternative, a substituted material portion electrode arrangement for ceramic metal halide lamps has been used. The most significant change involves the substitution of a flat molybdenum foil for a portion of the niobium or cermet rod in the sealing regions of the electrode tubes in electrode arrangements 35a and 35b of FIG. 1 as can be seen in the corresponding electrode arrangement, 35a′ or 35b′, shown in FIG. 2. In the full electrode arrangement view of FIG. 2A, a niobium rod, either 26a or 26b, is again provided in electrode arrangements 35a′ or 35b′ (could alternatively be molybdenum) but this rod is joined to the middle part molybdenum or cermet rod, either 29a or 29b, by a flat molybdenum foil, 36, also shown in cross section in FIG. 2B, welded to both the niobium and middle part rods. As before, the other end of the middle part rod is connected to the tungsten electrode rod, either 31a or 31b, by an annular space filling coil, either 34a or 34b, that is also wrapped therearound.
Molybdenum foil 36 forms a seal with the sealing frit, either 27a or 27b, and the polycrystalline alumina of the electrode tube, either 21a or 21b when positioned as one of the electrode arrangements shown in FIG. 1, and, to reduce thermal stresses, is chosen to be of a thickness less than 0.05 mm. Further reduction of stresses resulting from a right angles terminating edges in the sealing frit is obtained by beveling these edges to a point as shown for a beveled edge molybdenum foil, 36′, in the cross section view of FIG. 2C. A further measure taken to improve the mechanical and thermal properties of the molybdenum foil is doping with metal oxide particles such as yttrium oxide. Adding some surface roughness to the foil, as obtained for example by sand blasting or chemical etching, can also improve adhesion thereof to the frit during sealing.
However, electrode arrangements 35a′ or 35b′ of FIG. 2 require molybdenum foil 36 or 36′ to be wider than the diameter of the passageways extending through the preformed electrode tubes, either 21a or 21b, and the end closing disks, either 22a or 22b, of the structure of a typical size commonly used for arc discharge tube 20 if sufficient electrical current carrying capability is to be provided by that foil for the allowed thickness thereof. The diameter of these passageways cannot be increased because that implies the outer diameter of the electrode tubes would also have to increase to maintain sufficient tube wall thickness thereby increasing the thermal masses of these electrode tubes which would either alter the operating regime for arc discharge tube 20 or require a redesign thereof. As a result, use of electrode arrangements 35a′ or 35b′ of FIG. 2 necessitates proving slits across from one another in the walls of each of electrode tubes 21a and 21b to accommodate therein molybdenum foils 36 or 36′ if the structure of the commonly used for arc discharge tube 20 is to be retained. Thus, there is a desire for an electrode arrangement to be in arc discharge tube 20 that does not require a cost increasing modification of the commonly used structure for this discharge tube.